1. Field of the Invention
The present invention relates to an electron beam exposure mask and a method of manufacturing the same, and an electron beam exposure method.
2. Description of the Prior Art
Recently, an integration density of semiconductor integrated circuits (IC) have been improved more and more, and functions of the ICs have been increased. Thus, in the fields of industry such as computers, communications, and mechanical control where the ICs are employed, progress of technology has been expected widely. There exists some ICs such as DRAMS wherein a fourfold increase in integration density has been achieved in the past two or three years. Such high integration can be attained on the basis of the progress in fine pattern technology.
In an electron beam exposure technology, fine patterns of less than 0.05 .mu.m can be obtained if an alignment accuracy of an exposure mask can be achieved within less than 0.02 .mu.m. But, in such cases, it has been considered that such fine patterns cannot be employed in mass production of the LSI because of low throughput. However, in recent years, the throughput of about two sheets per hour has been realized by using a block exposure scheme or a blanking aperture array (BAA) scheme.
In such cases, a superfine pattern formation scheme, wherein both a pattern width and a pattern distance can be formed to be less than 0.20 .mu.m, is required for lithography technology used for manufacturing the semiconductor devices.
In case a resist is exposed to form such superfine patterns, a positive type resist is often advantageous compared to a negative type resist. In the negative type resist, a crosslinking reaction is caused in the resist material by an irradiation of the electron beam. Since the negative type resist becomes inflated by absorbing a developer in developing process to thus make the superfine patterns swell by the development process, dimensional accuracy of the superfine patterns is not assured. On the contrary, in the positive type resist, the crosslinked resist material is cut off by the electron beam or the energy beam. Since the positive type resist in the region where the electron beam is irradiated is melted down, the dimensional accuracy of the superfine patterns is readily assured.
Even if patterns of gate electrodes and capacitor electrodes of the transistor are exposed by employing a so-called "block exposure" scheme wherein the electron beam is shaped by means of a transmission mask formed of silicon, the dimensional accuracy of the gate electrodes can be easily obtained and surface areas of the capacitor electrodes can be formed widely, when they are exposed on the positive type resist. Since a capacitor is used as a charge storage capacitor of a memory device, for example, a large size capacitor is preferable.
In case the patterns of the gate electrode and the capacitor are formed in the block exposure transmission mask (also referred to as "block mask" hereinafter) used for exposing the positive type resist, the capacitor must be patterned to be surrounded by an opening portion 101 formed in a shape of a ring, as shown in FIG. 1A. Also, the gate electrode of the transistor must be patterned to be surrounded by an opening portion 102 formed like a U-shape, as shown in FIG. 1B. Now the pattern 101a surrounded by the ring opening portion 101 is referred to as an island-like pattern hereinafter, and the pattern 102a surrounded by the U-shaped opening is also referred to as a peninsula-like pattern or a tongue-like pattern hereinafter.
However, since the island-like pattern 101a surrounded by the ring opening portion 101 is formed in the air as it is, it is not of practical use. In addition, the peninsula-like pattern 102a surrounded by the U-shaped opening portion 102 has small mechanical strength.
Therefore, the island-like pattern 101a and the peninsula-like pattern 102a are supported by narrow crossbeams (referred to as bridging portions hereinafter) which can not shield the exposure electron beam, or otherwise, as shown in FIGS. 2A and 2B, the resist is exposed with 2 or more shots using plural masks 103 and 104, each composed of a plurality of divided opening patterns. Such a technique is disclosed in Unexamined Patent Publication (KOKAI) 59-222840 (EP Patent Application No. 83105177.6 filed on May 25, 1983), for example. However, it is apparent that, if this technique is employed, the throughput is inevitably decreased to about half or less.
Further, when lattice-like meshes formed by a narrow wire not exposed is used, some of the meshes can be covered by a thin film so as not to transmit the electron beam. As a result, the exposure mask having the island-like patterns and the peninsula-like patterns thereon can be formed in the exposure mask.
Furthermore, the exposure mask used for forming the patterns by using a diffraction effect of light is disclosed in Patent Application Publication (KOKOKU) 61-34667. In this exposure mask, as shown in FIG. 3A, "a plurality of small holes 105 are so arranged thickly that light waves diffracted by adjacent small holes 105 are overlapped with each other on the resist" to thus form a desired pattern.
According to the above exposure mask using the meshes and the above exposure mask using the plurality of small holes to form the desired pattern, one desired pattern is formed by the plural opening portions, and the lights are diffracted into rear sides of the crossbeams dividing these opening portions. Thereby, the patterns corresponding to the crossbeams are not resolved substantially on the resist which is formed on the wafer.
If the exposure mask is used to pass or transmit the light such as ultraviolet rays and X-rays, no significant problems occur. However, if an ionizing radiation such as an electron beam with high energy is irradiated onto the exposure mask, it causes the exposure mask to have a high temperature so that the exposure mask will be melted or elongated.
In particular, in order to improve the throughput of the electron beams exposure scheme, if an irradiation time is shortened by increasing the electron beam current density up to 40 A/cm.sup.2 or more, or otherwise if the electron beam accelerated more than 40 kV or more is employed, the above meshes cannot be employed as the exposure mask for the charged particles. In general, as the exposure mask for the charged particles such as the electron beam, it has been proposed that the silicon plate is used as the material and that hole portions are opened by anisotropic etching and trench etching techniques. Since the silicon plate is more stiff than both a thin metal plate and a crystal structure, the exposure mask made of silicon is not melted by the energy beam.
When, using the exposure mask made of the silicon substrate, the island-like patterns and the peninsula-like patterns described above are exposed, the following problems can occur.
(1) Since heat is accumulated in non-opening regions (energy beam shielding regions) in the exposure mask, the heat must be radiated effectively. If the heat is accumulated in the exposure mask, the dimensional accuracy of the exposure mask is deteriorated due to thermal expansion, or destruction of the exposure mask is caused due to stress generated by the thermal expansion.
(2) The island-like patterns and the peninsula-like patterns in the exposure mask must have sufficient mechanical strength. Therefore, the frame portions dividing the exposure mask into the plural opening regions have to be formed widely. However, the crossbeam portions are resolved as the patterns on the resist if they are formed too widely, so that object of the frame portions cannot be attained.
(3) A layout of the crossbeam portions must be so designed that a pattern shape to be exposed can be formed precisely. In other words, as described above, after the exposure mask having the above meshes or small holes is exposed by the charged particles (electron beam), it has not been apparent how to obtain the resist patterns with high accuracy. For example, in the pattern wherein a plurality of regions divided by the meshes are covered selectively by the thin film, it is hard to attain the pattern shapes with high accuracy
Moreover, if, as shown in FIG. 3A, the plurality of holes are disposed merely in vertical and horizontal directions, the pattern 106 having a jagged edge is formed as shown in FIG. 3B, so that the precise pattern cannot be formed.
In addition, charged particles irradiated through the opening regions of the exposure mask are distributed unequally on the resist when they pass or transmit near the edges of the opening regions. Thus this causes the degraded pattern accuracy. Note that this phenomenon is not restricted to the opening regions surrounding the island-like patterns and the peninsula-like patterns.